Noise surveys

TACKLING NOISE

By Bhawani P. Pathak

Are your workers slowly developing noise induced hearing loss? You might not know it for years -- until the compensation claims start coming in.

Take a walk through your workplace. Does the noise in any area seem louder than busy city traffic? Do people have to raise their voices to talk to someone three feet away? Next, talk to people who work in the noisy areas. At the end of a shift, do a workers who drive home have to turn up the volume on their car radios louder than it was in the morning in order to be able to hear? Do any of the people who have been at the workplace for a long time find it difficult to communicate in a crowd or party situation where there are other sounds or many voices in the same room?

If the answer to any of those questions is yes, you may have a noise problem, and it’s necessary to do a formal assessment of noise exposure (or have an expert consultant do the job). In other words, it’s time to do a noise survey before workers start to suffer hearing loss. (It might be years before the workers’ compensation claims start to roll in, and then it’s too late for those workers.)

Permanent hearing loss is the main concern related to occupational noise exposure. It progresses constantly as noise exposure continues over a period of months and years. The hearing impairment is not noticeable until the damage is substantial enough to interfere with routine activities. At this stage, permanent and irreversible hearing damage, which worsens as noise exposure continues, has occurred.

When exposure to the noise stops, noise induced hearing loss does not get any worse, but the person does not recover any hearing sensitivity. As the employee ages, hearing may also worsen as "age related hearing loss" adds to the noise induced hearing loss. There is no medical cure for noise induced hearing loss.

NOISE SURVEYS

A noise survey is the first step in assessing -- and eventually controlling -- exposure to noise in the workplace. But, before taking field measurments, it is important to determine the type of information required. The person making the measurements must clearly identify a number of things before proceding:

* the purpose of the measurement (compliance with noise regulations, hearing loss prevention, noise control, reducing community annoyance);

* the pattern of noise through the day, whether it is continuous, intermittent, or consisting of sudden "impulse" noises; and

* the locations of people who may be exposed to the noise.

A noise survey involves measuring the noise level at selected locations throughout the workplace at various times of day to identify noisy areas. This measurement is usually done with an instrument called a sound level meter, or SLM. By taking many measurements in different spots around a noise source, you can make a reasonably accurate identification of the locations of workers and the levels of noise to which they are exposed.

These noise level measurements are marked on a sketch map of the workplace. The greater the number of measurements taken, the more accurate the survey map becomes. A noise contour map can be produced by drawing lines on the sketch between points of equal sound level (similar to topographic maps which match areas of equal land height).

Such a noise survey map provides very useful information that enables you to do two things: identify noise hazard locations where employees are most likely exposed to noise above established exposure limits; and identify employees who may need to have their noise exposure measured for individual exposure levels (personal dosimetry).

EMPLOYEE EXPOSURE

The actual amount of noise an individual is exposed to depends on the workplace noise levels (how loud the noise is) and the exposure times (how long the individual is exposed to noise). Risk of hearing loss depends on these two factors. The need for individual noise exposure data arises when the initial noise surveys indicate a possibility that an employee’s exposure may exceed regulated noise exposure limits or the limits set by your company.

Determining a value for an individual’s noise exposure may not be as straightforward as it seems. If an employee works at the same location during the entire shift and the noise level at that location remains steady, his or her daily noise exposure level can be considered to be the same as the noise level at that location. However, if an employee works or moves to areas of varying noise levels, and uses different tools or machines for various times, the noise exposure has to be calculated as an average (called a "time weighted average", or TWA) of the noise levels to which he or she is exposed during the shift.

For example, consider a worker who is exposed to three major sources of noise during a given, eight-hour shift: Two hours (total) of using a hand grinder that produces 103 dB of noise; two hours with a 98 dB power saw; and four hours exposure to the general background noise of the plant, at 85 dB. A dosimeter set to the "three dB rule" would perform two simultaneous calculations in arriving at an eight-hour time weighted average noise exposure figure. First, it would average the noise over eight hours (the two hours’ exposure to 103 dB, for example, is equivalent to eight hours’ exposure to 97 dB). Second, it would add the various exposures together using the formula for adding noise levels, and arrive at an eight-hour TWA of 98 dBA.

Personal noise exposure of employees is performed using a noise dosimeter worn by the employee during the entire shift. The reliability of the noise data will depend on the employee’s cooperation, so it’s necessary to inform the affected employees about the purpose of the measurement and to emphasize the importance of wearing it during the entire measurement period.

INSTRUMENTS

The most common instruments used for measuring noise are the SLM, the integrating sound level meter (ISLM) and the dosimeter.

The SLM reads the sound pressure level (or noise) at one instant in a particular location. A "Type 2" SLM is sufficiently accurate for industrial field evaluations. (The more accurate and much more expensive Type 1 SLMs are primarily used in engineering, laboratory and research work.) Any SLM that is less accurate than a Type 2 should not be used for workplace noise measurement. An A-weighting filter, which causes the instrument to respond less to absolute sound levels and more to those frequencies that most affect human hearing, is generally built into all SLMs and can be switched on or off. Some Type 2 SLMs provide measurements only in dBA, meaning that the A-weighting filter is on permanently.

An integrating sound level meter (ISLM) is a hand-held instrument like the SLM. It measures equivalent sound levels over a period of time at a particular location. It yields a single reading of a given noise level, even if the actual sound level of the noise changes continually with time.

A noise dosimeter is a small, light-weight device that clips to a person’s belt and has a small microphone that fastens to the person’s collar near the ear. The dosimeter stores the constantly changing noise level information and carries out an averaging process. It is useful in industry where noise level varies during the work shift and/or when the wearer works at different locations with varying noise levels. A noise dosimeter gives an accurate picture of how much noise, from all sources, the wearer is actually exposed to over the course of the working day.

Dosimeters give readings for equivalent sound or noise level (Leq), maximum sound pressure level and elapsed time.Wearing the dosimeter over a complete work shift gives the time-weighted average noise exposure level and also the noise dose, expressed as a percentage, for the person wearing it. This is a highly individual result because of the differences in work habits between workers.

A comprehensive noise survey project, starting with a general noise survey and progressing to individual dosimetry if required, is really the only way to go from the general suspicion that there may be excessive noise exposure to the point at which hard data establishes whether there is a real problem or not. Only then can noise control and hearing protection programs be established that will tackle noise problems on anything more than a hit and miss basis.

Bhawani P. Pathak PhD, CIH ROH is with the Canadian Centre for Occupational Health and Safety in Hamilton, Ont.

BOX

HOW MUCH IS TOO MUCH?

Different provinces, territories and the federal jurisdiction use very different standards for setting the noise limits in their health and safety regulations. While regulations in different places can normally be expected to differ slightly, the actual difference in legally allowable noise varies hugely across Canada: Ontario, Quebec and New Brunswick still allow at least twice as much noise exposure as do British Columbia, Prince Edward Island and Newfoundland. The other jurisdictions are somewhere in between.

The discrepancy arises from the use of two different methods for calculating noise, and from differing opinions as to where to set the upper limit of allowable noise. This upper limit for an eight-hour shift, or "criterion level" is set at 90 dBA for many jurisdictions, 85 for some and 87 for others. (Because of the logarithmic scale used to measure noise, 90 dB is at least twice as much noise as 85 dB.) Then there is the "exchange rate" or the method of calculating how much louder a noise has to be to be "twice as much": some use three dB, others use five.

One method is based on the "equal energy principle" and is known as the three dB rule, three dB exchange rate or three dB doubling rate. When using this method, an exposure to 93 dBA for four hours is equivalent to 90 dBA exposure for eight hours, since the amount of noise is considered to double with every three dB increase in noise level. The underlying principle of this method is that hearing damage depends on the total energy entering the ears. The TWA, or time weighted average obtained using this method is called "Leq".

An alternative method of determining the TWA uses a concept introduced by OSHA (the Occupational Safety and Health Administration) in the United States. Under this method, a 95 dBA exposure for four hours is considered equivalent to 90 dBA exposure for eight hours. This method is also known as five dB rule, five dB exchange rate, or five dB doubling rate, since the total is taken to double with every five dB increase. Sometimes, the five dB TWA is called the "LOSHA" number .

When the noise level is steady throughout the work shift, the time weighted average is the same whether you are using the three dB and five dB rule(s). For variable or intermittent noise, however, the three dB rule results in a higher TWA than five dB rule. This has implications when complying with regulated noise exposure limits. For the same workplace noise, the five dB rule may indicate compliance with the exposure limit while the three dB rule may show that the exposure limit is exceeded.

For example, 105 dBA noise exposure for 60 minutes will give an eight-hour TWA of 90 dBA in the five dB exchange rate. The same noise exposure is equivalent to an eight-hour TWA (Leq) of 96 dBA in the three dB exchange rate.

Most experts recognize the three dB rule as more logical. The ISO 1999-1990 standard recommends the three dB rule for estimating noise induced hearing loss risk.

Noise exposure guidelines and standards are typically given as the maximum time of exposure permitted for various noise levels. The following chart shows the criterion levels (maximum allowable level for an eight-hour day) and exchange rates (the rate at which noise is taken to double) used by different Canadian jurisdictions.

Jurisdiction Maximum Permitted Exposure Level for 8 Hours in dBA Exchange Rate in dBA Maximum Peak Pressure Level Maximum Number of Impacts

Canada (Federal) 87 3* - -

British Columbia 85 3 135 -

Alberta 85 5 140 100

Saskatchewan 85 NS** - -

Manitoba 90 3 - -

Ontario 90 5 140 100

Ontario (draft) 90 3 140 -

Quebec 90 5 140 100

New Brunswick 85 5 140 100

Nova Scotia 85 5 140 100

Prince Edward Island 85 3 140 -

Newfoundland 85 3 - -

Northwest Territories 85 3 140 140

Yukon Territories 87 3 140 90+

** Not specified

*When the three dB exchange rate is used, there is generally no separate regulation for impulse/impact noise. Equivalent noise level is used to record impulse noise in the same way as that for continuous or intermittent noise.

The regulations or guidelines in several jurisdictions treat impulse noise separately from continuous noise. A common approach is to limit the number of impulses at a given peak pressure over a workday. The exact figures vary slightly, but generally the regulations permit 10,000 impulses at a peak pressure of 120 dB; 1,000 impulses at 130 dB; and 100 impulses at 140 dB. These values are based on military studies, using gunshots as the impulse noise source. They rate the effects of the impulse noise alone, with little or no continuous background noise. Alternatively, using a three dBA exchange rate, impulse noise can be considered jointly with any continuous noise, using the permissible exposure durations.

In addition to permissible exposures, many Canadian noise regulations also include requirements for engineering controls and hearing protection. You should consult your local jurisdiction for requirements that apply in your province.

BOX

TERMS OF THE TRADE

Noise: This is simply unwanted sound. Sound originates from a source that produces pressure waves in the surrounding air (or other medium) as a result of rotatory motion, vibration, friction or other mechanical processes. From this source, the sound waves spread out as a series of pressure fluctuations in the atmospheric air (similar to ripples on a lake when you drop a stone in the water).There are the two basic components of noise that can be measured: frequency and sound pressure.

Frequency: This is the number of cycles of vibration a noise source completes in one second. It determines the quality of sound. We hear frequency as pitch. Low pitched or bass sounds are low frequency sounds. High-pitched or treble sounds are high frequency sounds. Frequency is measured in hertz (Hz) units. One Hz is equal to one cycle per second. One kiloHerz (kHz) is 1000 Hz. A healthy, young person can hear sounds in the range of 20 to 20,000 Hz. Human speech is mainly in the frequency range of 300 to 3,000 Hz.

Sound Pressure: The loudness of sound depends on the magnitude of pressure fluctuations produced by the sound wave. These pressure fluctuations are referred to as sound pressure. Sound pressure is measured in pascals (Pa). A healthy, young person can hear sound pressures as low as 0.00002 Pa. A normal conversation produces a sound pressure of 0.02 Pa. A gasoline-powered lawn mower produces about 1 Pa. The sound is painfully loud at levels around 200 Pa. Atmospheric pressure is about 101,000 Pa. Thus even the painfully loud noise represents a change of only 2 in 1000.

Decibels (dB): It is not practical to use sound pressure ranging from millionths of a pascal to hundreds of a pascal to express noise loudness. Sound pressure level, or loudness, is therefore measured in decibel (dB) units. This scale is more convenient because it compresses the scale of numbers into a manageable range. Sound pressure converted to the decibel scale is called sound pressure level. The zero of the decibel scale (0 dB) is the same as a sound pressure of 0.00002 Pa.

A-weighting (dBA): Workplace noise is measured in A-weighted decibel units written as dBA. The A-weighting serves two important purposes. First, it takes into account the fact that the human ear is not equally sensitive to all of the audible sound frequencies. An A-weighting filter mimics the sensitivity of the human ear to sounds of various frequencies. It de-emphasizes low frequency sounds (500 Hz and lower) as people do not hear this frequency, as well as high frequency sounds. Second, A-weighting balances and integrates sound energy at all frequencies and gives a single number as an overall measure of the effective "loudness" of workplace noise.

Impulse (or impact) noise: A sudden, loud burst or noise of short duration. The classic example is the sound of a gunshot.

TWA: The "time weighted average" is a measure of total noise exposure that takes into account the variations in sound level at different times of day and for different periods of time. The TWA essentally adds up all the noise exposure over the time period and averages it out.

Criterion level: The accepted noise exposure limit or maximum sound level, expressed as a TWA, that is permitted over an eight hour day for a 40 hour week. This varies from provinve to province (see box, "How Much Is Too Much").

Exchange rate: This is the formula for calculating how much louder a noise has to be to be considered "twice as loud". There are still two conflicting systems in use: the three dBA exchange rate, which considers loudness to double with a three dBA increase; and the five dBA excahnge rate, which doubles with an increase of five.

In a bottle washing and filling plant where a noise survey was conducted, the noise levels were known to be high during certain activities, but no one was sure whether or not it was a problem. Instantaneous noise measurements taken at one work station found noise levels varied between 63 decibels (dBA) and 114 dBA. This information suggested that there was the potential for excessive noise exposure. Because it was almost impossible to determine the true extent of the problem, the worker at that station was asked to wear a noise dosimeter (an instrument to measure total noise exposure over the whole shift). The results showed an average noise level of 97 dBA, and a total noise exposure of 270 per cent the legal maximum.

Noise measurement data from studies of situations similar to your own are very helpful in assessing the potential noise problem. The "noise levels data base", produced by the Canadian Centre for Occupational Health and Safety, in Hamilton, Ontario, provides a collection of measured noise levels for a wide variety of industrial situations.

There are many different sources of noise in any given workplace, and different workers are exposed to them for varying amounts of time through the day. Some examples of noise levels are as follows:

* a pneumatic chipper at one metre -- 115 dB;

* a hand-held circular saw at one metre -- 115 dB;

* an average textile room -- 103 dB;

* a newspaper press -- 95 dB;

* a typical power lawn mower at one metre -- 92 dB;

* the maximum allowable, eight-hour exposure in Ontario -- 90 dB;

* a diesel truck at 50 km per hour at 20 metres -- 85 dB;

* the maximum allowable, eight-hour exposure in British Columbia -- 85 dB;

* a passenger car at 60 km per hour at 20 metres -- 65 dB;

* a normal conversation at one metre -- 55 dB; and

* a quiet room -- 40 dB.

(table)

TIPS FOR INSTRUMENT SELECTION

Type of measurement Appropriate Instruments (in order of preference) Results obtained Comments

Personal noise exposure Noise levels generated by a particular source Impulse survey Impulse noise

Dosimeter Dose or equivalent sound level Most accurate for personal noise exposures

Integrating sound level meter (ISLM) Equivalent sound level If the subject is mobile, it may be difficult to determine personal exposure, unless work can easily be divided into defined activities

Sound level meter Instantaneous reading of sound level in dBA If noise levels vary considerably, it is difficult to determine average exposure. Only useful when work can be divided into defined activities and noise levels are relatively stable.

Sound level meter Instantaneous reading of noise level Measurement should be taken one to three metres from the noise source

Integrating sound level meter Equivalent sound level Particularly useful if noise is highly variable. Can measure equivalent sound level over a short period of time (one minute).

Sound level meter Instantaneous reading To produce noise map of an area; take measurements on a grid pattern

Integrating sound level meter Equivalent sound level For highly erratic noise

Impulse sound level meter Peak pressure (in dBA) To meeasuree the peak of each impulse noise

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